An existing SFP (Small Form-factor Pluggable) connector is a small form-factor pluggable interface for data communication and is widely used in switches over Ethernet to implement interconnection between switches and between switches and servers. The SFP connector can also connect optical fiber cables.
An SFP-DD (SFP Double Density) connector under development presently has the double density of SFP. The SFP-DD has more two rows (upper and lower rows) of contacts than the SFP, that is, a SFP-DD receptacle has more two rows of terminals than a SFP receptacle in a mating direction and forms a front two rows of contact portions which face in an up-down direction and a back two rows of contact portions which face in the up-down direction in the slot, and a SFP-DD plug has more one row of connection pads (actually, that is more one upper row of connection pads and one lower row of connection pads, because the one upper row of connection pads and the one lower row of connection pads face each other in the up-down direction, the one upper row of connection pads and the one lower row of connection pads are counted as one row of connection pads) than a SFP plug in the mating direction, thus the SFP-DD receptacle and the SFP-DD plug respectively have longer lengths than the SFP receptacle and the SFP plug in the mating direction. However, the SFP connector and the SFP-DD connector are similar in appearance, the SFP receptacle and SFP-DD receptacle have the same port sizes, when the SFP plug is inserted into the SFP-DD receptacle, because the number of the terminals of the SFP-DD receptacle is greater than the number of the connection pads of the SFP plug, the SFP-DD receptacle can also be compatible with the SFP plug. However, when the SFP-DD plug is inserted into the SFP receptacle, a half of the connection pads of the SFP-DD plug do not have corresponding terminals of the SFP receptacle to establish an electrical connection. In other words, the SFP receptacle cannot be compatible with the SFP-DD plug.